“The question you have posed is therefore entirely hypothetical, unlikely to occur, and one we hope no President will ever have to confront."
But the letter merely says that the president has "no intention" of carrying out drone strikes against US citizens on US soil without due process, not that the president cannot do this.
Holder was replying to questions from Rand Paul about John Brennan, who is the administration's nominee for CIA director. Given that neither Holder nor Brennan will rule out the use of drone strikes against American citizens on American soil, Rand Paul is presently--as of this writing--in his seventh hour of filibustering the vote to confirm Brennan.
Senator Paul knows that he doesn't have the votes to block the confirmation, but he's certainly reminding us what can happen when a man stands on principle. The amazing thing is that the principle--that no president, no administration, no CIA and no governmental agency should have the right to strike against United States citizens on American soil without due process--should be one with broad bipartisan support, not one that leads all but those most concerned about civil liberties to shrug and dismiss the issue.